Coated steel strips, methods of making the same, methods of using the same, stamping blanks prepared from the same, stamped products prepared from the same, and articles of manufacture which contain such a stamped product

ABSTRACT

Coated steel having a well defined and uniform thickness is useful and advantageous for preparing products by hot stamping. Products prepared by hot stamping such a steel are particularly advantageous when subjected to a subsequent spot welding step.

FIELD OF THE INVENTION

The present invention relates to, among other things, coated steels,methods of making such coated steels, including hot dipping, methods ofusing such coated steels, stamping blanks prepared from coated steels,stamped products prepared from coated steels, and to various uses of theinvention products such as in spot welding, etc.

Additional advantages and other features of the present invention willbe set forth in part in the description that follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from the practice of thepresent invention. The advantages of the present invention may berealized and obtained as particularly pointed out in the appendedclaims. As will be realized, the present invention is capable of otherand different embodiments, and its several details are capable ofmodifications in various obvious respects, all without departing fromthe present invention. The description is to be regarded as illustrativein nature, and not as restrictive.

BACKGROUND OF THE INVENTION

In recent years the use of pre-coated steels in hot-stamping processesfor the shaping of parts has become important, especially in theautomotive industry. Fabrication of such parts may include the followingmain steps:

-   -   Pre-coating of a steel sheets, by hot dipping    -   Trimming or cutting for obtaining blanks    -   Heating the blanks in order to obtain alloying of the steel        substrate with the pre-coating, as well as the austenitizing of        the steel    -   Hot forming followed by rapid cooling of the part in order to        obtain predominantly martensitic structures        See for Example U.S. Pat. No. 6,296,805, incorporated herein by        reference.

Thanks to an alloying of the pre-coating with the steel substrate, whichhas the effect of creating intermetallic alloys with high meltingtemperature, the blanks having such coating may be heated in atemperature range where austenitizing of the metallic substrate takesplace, allowing further hardening by quenching.

Heat treatments of the blanks in view of the intermetallic alloying ofthe coating and austenitizing of the substrate are most frequentlyperformed in furnaces, where blanks are traveling on rollers. Thethermal cycles experienced by the blanks include first a heating phasewhose rate is a function of parameters such as blank thickness, furnacetemperature, traveling speed, and coating reflectivity. After thisheating phase, thermal cycles generally include a holding phase, whosetemperature is the regulation temperature of the furnace. Problemshowever are experienced with the furnace operation: the rollers maybecome fouled by metallic deposits which come from the pre-coating ofthe blanks. If these deposits are excessive, maintenance of the rollershas to be performed and productivity is decreased.

Parts obtained after heating and rapid cooling display very highmechanical resistance and may be used for structural applications, forexample for automotive industry applications. These parts must befrequently welded with others and high weldability is required. Thismeans that:

-   -   The welding operation should be performable in a sufficiently        wide operating range in order to guarantee that an eventual        drift of the nominal welding parameters has no incidence on weld        quality. For resistance welding, which is very common in the        automotive industry, an operating welding range is defined by        the combination of parameters: welding current intensity I and        force F applied of the parts during welding being among the most        important. A proper combination of these parameters helps to        ensure that insufficient nugget diameter is not obtained (caused        by too low intensity or too low force) and that no weld        expulsion occurs.    -   The welding operation should also be performed in such a way        that high mechanical resistance is obtained on the weld. This        mechanical resistance may be evaluated by tests such as by        shear-tensile tests or cross-tensile tests.

There remains a need for coated steels which may be conveniently used toprepare shaped parts by a stamping process. There also remains a needfor coated steels which may be used to prepare shaped parts by astamping process which are suitable for welding. There also remains aneed for processes for preparing such coated steels and stamped parts.

SUMMARY OF THE INVENTION

The inventors have discovered that certain coated steels in which a basesteel strip is at least partially coated (sometimes termed “pre-coated,”this prefix indicating that a transformation of the nature of thepre-coating will take place during heat treatment before stamping) on atleast one side with a coating of either aluminum or an aluminum alloyand in which the coating has a defined thickness and is preferablysubstantially uniform, are conveniently formed into shaped parts afterheating by stamping and are conveniently welded. In addition, theinventors have discovered that the roller fouling problem describedabove generally arises from an insufficient degree of intermetallicalloying between the substrate and the metallic pre-coating.Furthermore, it was discovered that the location of the fouling of therollers corresponds to zones of the blanks in contact with the rollerswhere the metallic pre-coating thickness locally exceeds the averagethickness. While not bound by a particular theory, it is believed thatif a pre-coating is locally too thick, intermetallic alloying isinsufficient and the pre-coating melts, fouling the rollers. Thus, theinventors have discovered that the control of the homogeneity of thepre-coating thickness over the entire sheet within given tolerances isan important factor for obtaining the desired degree of intermetallicalloying, allowing one to improve the resistance to the subsequentmelting of the coating during travel on rollers.

The inventors have also discovered that particularly good weldability ofaluminized and hot stamped parts is associated with a special successionof coating layers on the parts, proceeding from steel substrateoutwards.

The inventors have also discovered that a specific combination oftransfer time between the heating furnace and the stamping die, theamount of deformation during stamping, the temperature of stamping, thecooling rate of the product during stamping, leads to the fabrication ofa part with a fully homogeneous martensitic structure and that anincrease in ductility or energy absorption of the parts after stampingis obtained by a reduction of sulfur below a critical value, these twobenefits being obtained with or without the invention aluminum/aluminumalloy coating, and with other coatings.

OBJECTS OF THE INVENTION

Accordingly, and in view of the above, it is one object of the presentinvention to provide novel pre-coated steels strips which may beconveniently processed into stamping blanks.

It is another object of the present invention to provide novelpre-coated steels strips or sheets which may be conveniently formed intoparts by stamping.

It is another object of the present invention to provide novel coatedsteels which may be conveniently formed into parts by hot stamping.

It is another object of the present invention to provide novel methodsfor making such a coated steel.

It is another object of the present invention to provide novel stampingblanks which are prepared from such a coated steel.

It is another object of the present invention to provide novel methodsof making such stamping blanks.

It is another object of the present invention to provide novel stampedparts which are prepared from such a coated steel.

It is another object of the present invention to provide novel methodsof making such stamped parts.

It is another object of the present invention to provide novel articlesof manufacture, such as a motor vehicle, which contain such stampedparts.

It is another object of the present invention to provide novel stampedparts.

It is another object of the present invention to provide novel methodsof making welded stamped parts.

It is another object of the present invention to provide novel articlesof manufacture, such as a motor vehicle, which contain such weldedstamped parts.

It is another object of the present invention to provide novel weldedcoated steels and welded stamping blanks.

It is another object of the present invention to provide novel methodsof making such welded coated steels and welded stamping blanks.

These and other objects, which will become apparent during the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a preferred embodiment of an invention coated part,after heat treatment and stamping. The succession of layers of thecoating on the steel substrate is: (a) interdiffusion layer; (b)intermediate layer; (c) Intermetallic layer; and (d) superficial layer.This arrangement is particularly favorable for the further welding ofthe part.

FIG. 2 illustrates a coating of a steel substrate after heat treatmentand stamping which does not correspond to the invention. This successionof layers (interdiffusion layer and intermetallic layer) yields inferiorresults in resistance spot welding.

FIG. 3 illustrates a microstructure of a steel part, hot stamped andcooled under conditions not according to the invention.

FIG. 4 illustrates the microstructure of a steel part, hot stamped andcooled according to a preferred set of conditions according to theinvention.

FIG. 5 illustrates the influence of sulfur on the bending angle of partsafter hot stamping.

FIG. 6 illustrates the influence of sulfur on the initiation energy offracture of parts after hot stamping.

FIG. 7 shows conditions of furnace temperature as a function of thetotal dwell time in the furnace for sheets of total thicknesses of from0.7-1.5 and 1.5-3 mm that provide particularly favorable coatings forwelding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As noted above, the inventors' discovery that certain coated steels, inwhich a base steel is at least partially pre-coated on at least one sidewith a coating of either aluminum or an aluminum alloy and in which thepre-coating has a defined thickness and is substantially uniform, areconveniently formed into shaped parts by stamping, forms one basis forthe invention.

In the context of the present invention, the terms first side (orside 1) and second side (or side 2) of the strip or sheet, etc., of basesteel refer to the two large, opposite-facing, surfaces which have asurface area defined by the length and width of the strip of base steel.In contrast, the side edges of the strip of base steel are the twosmall, opposite-facing, surfaces which have a surface area defined bythe length and thickness of the strip. The top and bottom edges of thestrip of base steel are the two small, opposite-facing, surfaces whichhave a surface area defined by the width and thickness of the strip. Inthe following, t_(p) designates the thickness of the pre-coating, at anyconsidered location on the sides 1 and 2 of a sheet or a blank. Inparticular, in the case of sheets coated on two sides 1 and 2, t_(p1)stands for the thickness on side 1, and t_(p2) for the thickness on side2.

According to a highly preferred embodiment t_(p) is controlled in aprecise range, expressed by (t_(pmin), t_(pmax)), in order to improvethe resistance to the fouling of the rollers. The thickness ispreferably controlled, both in the longitudinal (or rolling) directionof the strip or sheet, as well in the transverse direction.

In relation to the problem of the fouling of rollers, the control of thepre-coating thickness on the side of the sheet or blank which isdirectly in contact with the rollers is especially important. Becausedifferent operations can follow the step of coating the steel sheet (forexample hot-dip-coating which provides coated sides 1 and 2), it ispreferable to carefully control the pre-coating on both sides of thesheet. For example, after any of coiling, handling, cutting, punching,etc., sides 1 and 2 may be not readily identifiable. But when control oft_(p) is effected on the two sides of the sheet that has been coated(first side and second side) it is not necessary to track sides 1 and 2as neither side will foul the roller. In addition, it is not necessaryto trim the sheet in order to obtain a preferred smaller sheet having amore uniform homogeneity of pre-coating thickness, thus providing asheet that is pre-coated by, e.g., hot-dipping. In other words,important benefits are obtained when one controls the pre-coatingminimum and maximum thickness of the first side (t_(pmin1), t_(pmax1))and the pre-coating minimum and maximum thickness of the second side(t_(pmin2), t_(pmax2)) of the subject steel sheet or blank.

Hot-dip coated steels are the preferred steels herein. However,regardless of the coating method, the pre-coating thickness on one orboth sides of the sheet can be measured and monitored continuously on acoating line directly after the coating operation. This may be realizedby devices known per se, such as thickness gages relying on X-Rayabsorption. At every moment, the measurement of thickness at a givenlocation may be performed for example on an area of a few hundreds ofmm², this representing the dimension of the zone irradiated by X-Ray.

In a preferred embodiment, a plurality of such devices are positioned atdifferent distances in the transverse direction of the strip in order toobtain a profile of the thickness of the pre-coating along the stripwidth.

The inventors have discovered that the resistance to the pollution orfouling of the rollers in the furnaces is improved when the minimum andmaximum thickness respectively of at least one of the first side(t_(pmin1), t_(pmax1)) and the second side (t_(pmin2), t_(pmax2)) arerespectively equal to 20 and 33 micrometers (a micrometer is the same asa micron and is a metric unit of length equal to one millionth of ameter). In other words, in a preferred embodiment, at every location onat least one face of the sheet or the blank, the thickness t_(p) of thepre-coating is preferably from 20-33 micrometers in thickness, including21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 and 32 micrometers and allranges and subranges therebetween, and including all numbers betweeneach listed value as if written out in its entirety (e.g., 22.34micrometers). For hot-dip coating, precise control of this range ofpre-coating thickness can be realized on an operating line for exampleusing a system of nozzles blowing gas after coating, for example afterexit of the strip or sheet from a bath, and by the flatness of thestrip. Number, geometry and location of the nozzles, and flow rates areamong the main parameters for a precise control of the thickness t_(p).Given the present disclosure, one of ordinary skill in this art cancontrol the pre-coating thickness as described herein without unduelabor.

The invention refers to strips produced in industrial conditions, i.e.where control of pre-coating thickness is effective over a wide surfacestrip, i.e. with a length greater than 100 m, and with a width greaterthan 600 mm. In such a way, blanks cut or trimmed out these stripsdisplay very high homogeneity of pre-coating thickness, and the settingsof the heating treatments in the furnace do not have to be changed forbeing adapted to eventual variations of this thickness.

While not bound by a particular theory of operation, the inventorsbelieve that several of the benefits of the invention are related tothis pre- coating thickness range, such as the following:

For a pre-coating thickness less than 20 micrometers, the alloyed layerwhich is formed during the heating of the blank has an insufficientroughness. Thus, the adhesion of subsequent painting is low on thissurface, and the corrosion resistance is decreased.

If the pre-coating thickness is more than 33 micrometers at a givenlocation on a sheet, the risk is that the difference of thicknessbetween this location and some other locations where the pre-coating isthinner, becomes too important: The settings of the heating treatment inthe furnace may be adapted to the thinner value of the pre-coating, butnot to the thicker one. Thus, the alloying reaction forming theintermetallic alloy may take place to an insufficient degree since themean diffusion distance of the elements in the pre-coating becomessignificantly less than the local value of the pre-coating thickness. Asa matter of results, alloying will become much more difficult in theexternal (or superficial) part, particularly in the case of high heatingrate.

Thus, in a first embodiment, the present invention provides certaincoated steel strips, which comprise a strip of base steel and apre-coating of aluminum or an aluminum alloy on at least a part of oneside of the strip of the base steel. For many applications, the strip ofbase steel may comprise any type of steel which may be coated witheither aluminum or an aluminum alloy. However, for certain applications,such as a structural part of an automobile, it is preferred that thestrip of base steel comprise an ultra high strength steel (UHSS). Insuch cases, it is particularly preferred that the strip of base steelcomprises a boron steel.

The inventors have also discovered that good welding results areachieved if the coating obtained on parts made out of blanks havingundergone intermetallic alloying, austenitizing and hot stamping,displays distinctive features. It must be pointed out that this coatingis different from the initial pre-coating, since the thermal treatmentcauses an alloying reaction with the steel substrate which modifies boththe physico-chemical nature and the geometry of the pre-coating: in thisregard, the inventors have discovered that particularly good weldabilityof aluminized and hot stamped parts is associated with the followingsuccession of coating layers on the parts, proceeding from steelsubstrate outwards:

-   -   (a) Interdiffusion layer,    -   (b) Intermediate layer,    -   (c) Intermetallic layer,    -   (d) Superficial layer        See, e.g., FIG. 1. In a preferred embodiment these layers are as        follows:    -   (a) Interdiffusion layer, preferably with medium hardness (e.g.,        HV50g between 290 and 410, HV50g designating the hardness        measured under a load of 50 grams) In a preferred embodiment        this layer has the following composition, by weight: 86-95% Fe,        4-10% Al, 0-5% Si    -   (b) Intermediate layer (HV50g around 900-1000 e.g., +/−10%)) In        a preferred embodiment this layer has the following composition,        by weight: 39-47% Fe, 53-61% Al, 0-2% Si    -   (c) Intermetallic layer, with hardness HV50g around 580-650,        e.g., +/−10%) In a preferred embodiment this layer has the        following composition, by weight: 62-67% Fe, 30-34% Al, 2-6% Si    -   (d) Superficial layer (HV50g around 900-1000 e.g., +/−10%)) In a        preferred embodiment this layer has the following composition,        by weight: 39-47% Fe, 53-61% Al, 0-2%        In a preferred embodiment the total thickness of layers (a)        to (d) is greater than 30 micrometers.        In another preferred embodiment, the thickness of layer (a) is        less than 15 micrometers, for example 14, 12, 10, 8, 6, 4, 2, or        1 micrometers, and all whole numbers, ranges and subranges        therebetween, and including all numbers between each listed        value as if written out in its entirety (e.g., 13.84        micrometers).

The inventors have discovered that high weldability is especiallyobtained when layers (c) and (d) are essentially continuous (i.e.:occupying at least 90% of the level corresponding to the consideredlayer) and when less than 10% of layer (c) is present at the extremesurface of the part. Without being bound by a theory, it is thought thatthis particular layer disposal, in particular layer (a) and, layers (c)and (d) influence the resistivity of the coating both by their intrinsiccharacteristics and by the effect of roughness. Thus, current flow, heatgeneration at the surfaces, and nugget formation in the initial stage ofspot welding are affected by this particular arrangement.

This favorable layer disposition is obtained for example when aluminum-or aluminum alloy pre-coated steel sheets, whose thickness range from,e.g., 0.7 to 3 mm, are heated for 3 to 13 minutes (this dwell timeincludes the heating phase and the holding time) in a furnace heated toa temperature of 880 to 940° C. Other conditions leading to suchfavorable layer dispositions are found in FIG. 7:

For sheets of total thicknesses greater or equal to 0.7 mm, and lessthan or equal to 1.5 mm, the preferred treatment conditions: (furnacetemperature, total dwell time in the furnace) are illustrated in FIG. 7by conditions lying within the limits of diagram “ABCD”

For sheets of total thicknesses greater than 1.5 mm, and less than orequal to 3 mm, the preferred treatment conditions: (furnace temperature,total dwell time in the furnace) are illustrated in FIG. 7 by diagram“EFGH”.The heating rate V_(c) is comprised between 4 and 12° C./s for producinga favorable alloyed layer disposition. In this regard, this “heatingrate” reflects the temperature rise which is experienced by thepre-coated steel upon being located in the preheated furnace. V_(c) isdefined as the mean heating rate between 20 and 700° C. The inventorshave discovered that the control of V_(c) in this precise range is a keyfactor, because it controls directly the nature and the morphology ofthe alloyed layers which are formed. It is here underlined that theheating rate V_(c) is different from the mean heating rate, which is theheating rate between room temperature and furnace temperature. The ratesof 6, 7, 8, 9, 10, and 11° C./s are included as are all numbers, rangesand subranges therebetween, and including all numbers between eachlisted value as if written out in its entirety (e.g., 7.7° C./s). Inthis regard, all of the conditions specified within FIG. 7 areincorporated herein by reference thereto. Particularly preferredconditions are:

(for thicknesses of 0.7-1.5 mm)

-   -   930° C., from 3 minutes up to 6 minutes;    -   880° C., from 4 minutes 30 seconds up to 13 minutes (for        thicknesses of 1.5 to 3 mm)    -   940° C., from 4 minutes up to 8 minutes;    -   900° C., from 6 minutes 30 seconds up to 13 minutes        A special advantage arises from pre-coatings whose thickness is        comprised between 20 and 33 micrometers, since this thickness        range yields favorable layer disposal, and since the homogeneity        of the pre-coating thickness is associated to an homogeneity of        the coating formed after alliation treatment.

Heated blanks are thereafter transferred to a die, hot stamped to obtaina part or product, and cooled at a rate of more than 30° C./s. Thecooling rate is defined here as the mean rate between the exit of theheated blank from the furnace, down to 400° C.

The strip of base steel is coated with either aluminum or an aluminumalloy. Commercially pure aluminum is known in the art as Type 2aluminum, while alloys of aluminum with 5 to 11% by weight of siliconare known in the art as Type 1 aluminum. Silicon is present in order toprevent the formation of a thick iron-metallic intermetallic layer whichreduces adherence and formability. Other alloying elements useful withaluminum herein include iron, between 2.5 and 3% by weight, and calcium,between 15 and 30 ppm by weight, including combinations of two or morethereof with aluminum.

A typical metal bath for an Al—Si coating generally contains in itsbasic composition by weight, from 8% to 11% silicon, from 2% to 4% iron,the remainder being aluminum or aluminum alloy, and impurities inherentin processing. Typical composition of Al—Si coating is: Al-9.3% Si-2.8%Fe. Invention coatings are not limited to these compositions, however.

The strip of base steel used herein may be any which can be coated by aconventional coating technique. For example, the strip of base steel maybe any hot rolled strip, such as those prepared by hot rolling a steelslab (either with or without subsequent cold rolling). Typically, thestrip of base steel will be stored and transported in the form of a coilboth before and after the formation of the coating.

An example of a preferred steel for the strip of base steel is onehaving the following composition by weight:

-   -   0.10%<carbon<0.5%    -   0.5%<manganese<3%    -   0.1%<silicon<1%    -   0.01%<chromium<1%    -   titanium<0.2%    -   aluminum<0.1%    -   phosphorus<0.1%    -   sulfur<0.05%    -   0.0005%<boron<0.010%,        the remainder comprising, consisting essentially of, or        consisting of iron and impurities inherent in processing. Use of        such a steel provides a very high mechanical resistance after        thermal treatment and the aluminum-based coating provides a high        resistance to corrosion.

Particularly preferably, the composition by weight of the steel in thestrip of base steel is the following:

-   -   0.15%<carbon<0.25%    -   0.8%<manganese<1.8%    -   0.1%<silicon<0.35%    -   0.01%<chromium<0.5%    -   titanium<0.1%    -   aluminum<0.1%    -   phosphorus<0.1%    -   sulfur<0.05%    -   0.002%<boron<0.005%,        the remainder comprising, consisting essentially of, or        consisting of iron and impurities inherent in processing.

A preferred strip herein is 100 m long and 600 mm wide. Preferredthicknesses are 0.7 to 3 mm.

Even more preferably, in the composition by weight of the sheet, theweight ratio of titanium content with respect to the nitrogen content isin excess of 3.42, believed to be a level at which the boron is nolonger able to combine with the nitrogen.

An example of preferred commercially available steel for use in thestrip of base steel is 22MnB5.

Chromium, manganese, boron and carbon may be added, in the compositionof the steel according to the invention, for their effect onhardenability. In addition, carbon makes it possible to achieve highmechanical characteristics thanks to its effect on the hardness of themartensite.

Aluminum is introduced into the composition, to perform deoxidation inthe liquid state and to protect the effectiveness of the boron.

Titanium, the ratio of the content of which with respect to the nitrogencontent should be in excess of 3.42, is introduced for example in orderto prevent combining of the boron with the nitrogen, the nitrogen beingcombined with titanium.

The alloying elements, Mn, Cr, B, make possible a hardenability allowinghardening in the stamping tools or the use of mild hardening fluidslimiting deformation of the parts at the time of thermal treatment. Inaddition, the composition according to the invention is optimized fromthe point of view of weldability.

The steel in the sheet may undergo a treatment for globularization ofsulfides performed with calcium, which has the effect of improving thefatigue resistance of the sheet.

As mentioned above, ultra-high-strength can be provided with the steelsheet coated and hot stamped according to the invention. This high levelof strength is sometimes associated with a limited ductility. Inapplications requiring a higher ductility, in particular when an abilityfor bending is required from a part or a product, the inventors havediscovered that increased ductility can be obtained if sulfur isparticularly controlled: when the sulfur level of base steel is lowerthan or equal to 0.002% (20 ppm), the bending angle can be greater than60° and enhanced ductility and tearing resistance are obtained on theparts having experienced heat treatment and stamping. Preferred levelsinclude 20, 18, 15, 13, 10, 8, 5, 2, etc ppm sulfur. In fact, thisbenefit applies to steels in general, and is not limited to coatedsteels or to steels coated with Al or Al alloy coatings. While not boundby a particular theory, when analyzing causes of premature failure ofsome parts in bending operations, the inventors observed that failureinitiated on sulfide inclusions. It is thus believed that decohesionsbetween inclusions and the martensitic or bainito-martensitic matrix actas stress-concentration factors and trigger further crack propagation inthe ductile mode.

The invention also concerns a process for producing a part starting froman invention coated sheet, then cut into a blank which, after shaping,the coating of the blank is subjected to an increase in temperature at aspeed in excess of 4° C./second, but lower than 12° C./second. Theheating rate V_(c) is defined as the mean rate between 20 and 700° C.

The invention also concerns the use of a hot-rolled steel sheet whichthen can be cold-rolled and coated, for structural and/or anti-intrusionor substructure parts for a land motor vehicle, such as, for example, abumper bar, a door reinforcement, a wheel spoke, etc.

The sheet according to the invention described above can derive, byreason of its processing, from a hot-rolling mill, and possibly may becold-rerolled again depending on the final thickness desired. It then iscoated with an aluminum-based coating, for example by dipping in a bathcontaining, in addition to the aluminum source/alloy, e.g., from 8% to11% silicon and from 2% to 4% iron, the sheet having a high mechanicalresistance after thermal treatment and a high resistance to corrosion,as well as a good capacity for painting and gluing.

The coating is preferably controlled as above, and has in particular thefunction of protecting the basic sheet against corrosion in variousconditions. The thermal treatment applied at the time of a hot-formingprocess or after forming makes it possible to obtain high mechanicalcharacteristics which can exceed 1500 MPa for mechanical resistance and1200 MPa for yield stress. The final mechanical characteristics areadjustable and depend in particular on the martensite fraction of thestructure, on the carbon content of the steel and on the thermaltreatment. At the time of thermal treatment performed on a finished partor at the time of a hot-shaping process, the coating forms a layerhaving a substantial resistance to abrasion, wear, fatigue, shock, aswell as a good resistance to corrosion and a good capacity for paintingand gluing. The coating makes it possible to avoid differentsurface-preparation operations such as for steel sheets for thermaltreatment not having any coating.

The steel sheet can be pre-coated by dipping, after pickling, in analuminum bath containing for example only aluminum or either aluminumand from 8% to 11% silicon and 2% to 4% iron, or only from 2% to 4%iron, or even in an aluminum bath preferably containing from 9% to 10%silicon and 2% to 3.5% iron. The aluminum may be aluminum per se or analuminum alloy.

In an example of implementation of a coating of the sheet by dipping ina metal bath containing an aluminum alloy comprising a proportion ofapproximately 90% aluminum, the coating layer comprises a first alloylayer in contact with the surface of the steel. This layer, directly incontact with the surface of the sheet, is highly alloyed with iron.

A second coating layer, on top of the first, contains approximately 90%aluminum and may contain silicon and a small amount of iron, dependingon the composition of the bath.

The first alloy layer may crack when the sheet is submitted to highstrains during cold forming operations of the manufacture of parts.

According to the invention, after the forming of the part, the coatingis subjected to an increase in temperature at a speed in excess of 4°C./second. This rise in temperature makes possible a rapid remelting ofthe aluminum which fills in the cracks generated by the operation ofshaping of the part.

At the time of thermal treatment, the base coating, of aluminum forexample, is transformed into a layer alloyed with iron and comprisingdifferent phases depending on the thermal treatment and having aconsiderable hardness which may exceed 600 HV50 g.

Another advantage of the invention lies in the fact that the diffusionof the iron in the coating is initiated at high temperature. One thuswill have a better cohesion between coating and steel in the sheet. Inanother form of the invention, the thermal treatment may be performedlocally, in heavily deformed zones.

According to the invention, the sheet, in the delivery state in a coilor in a is sheet, the thickness of which may range between 0.25 mm and15 mm, has good forming properties and a good resistance to corrosion aswell as a good capacity for painting or gluing. Preferably, the steelsheet or blank has a thickness less than 3 mm, since the cooling ratesthat may be achieved after quenching are high and help to obtainmartensitic structures.

The steel sheet, a coated product, has a substantial resistance tocorrosion in the delivery state, during forming and thermal treatmentsas well as during usage of the finished part.

The presence of the coating at the time of thermal treatment of theparts makes it possible to prevent any decarburization of the base metalas well as any oxidation. That is an undeniable advantage, in particularin the case of hot forming. Furthermore, heating of the treated partdoes not require a furnace having a controlled atmosphere to prevent adecarburization.

Thermal treatment of the metal in the sheet comprising heating at atemperature ranging between Ac1, starting temperature of austenitictransformation when heating, for example 750° C. and 1200° C., in afurnace, for a period which depends on the temperature to be reached andthe thickness of the blank. The composition is optimized so as to limitthe grain growth at the time of thermal treatment. If the structuresought is completely martensitic, the holding temperature should be inexcess of Ac3, for example 840° C., temperature of complete austenitictransformation. The temperature holding should be followed by a coolingadjusted to the final structure sought.

Blanks are thereafter transferred from the furnace to a stamping press.When the elapsed time between the exit of the blanks from the furnaceand the introduction in the stamping press is more 10 seconds, a partialtransformation from austenite is susceptible to appear: if obtaining afull martensitic structure is desired, the transfer time between theexit of the furnace and stamping should be less than 10 s.

The inventors have also discovered that the obtaining of a fullymartensitic structure is linked to the amount of deformation in the hotforming operation: the amount of local deformation caused by hot formingis closely linked to the shape of the part or product and may exceedlocally 40 or 50% in some particular regions. The inventors found that,when the local strain exceeds a critical value of 10%, the cooling ratemust be sufficiently high in order to get a total martensitictransformation. Otherwise, bainitic transformation can take place at asignificant amount instead of martensitic transformation. Thus, the riskis that heterogeneous structure appears on parts with complex shapewhere some locations are much more deformed than others. In thisrespect, the inventors put into evidence that, on the locations of theparts where the forming strain is higher than 10%, the cooling rate mustbe increased beyond 50° C./s in order to guarantee full martensitictransformation. The cooling rate is defined as the mean rate between theexit of the heated blank from the furnace, down to 400° C.

But one may also seek to obtain ferrito-bainitic or ferrito-martensiticstructures, by a heating at a temperature ranging between Ac1, forexample 750° C. and Ac3, for example 840° C., followed by an appropriatecooling. According to the level of resistance to be achieved and thethermal treatment applied, one or several of these constituents is/arepresent in variable proportions.

The modulation of thermal treatment parameters makes it possible toachieve, with a given composition, different levels of hot and coldsheet resistance according to the thickness sought. For the highestresistance levels, the structure is composed predominantly ofmartensite.

The steel is particularly suited to the production of structural andanti- intrusion parts.

The invention thus enables one to produce a hot- or cold-rolled steelsheet of a desired thickness, coated, and affording extensive formingpossibilities and which, after thermal treatment performed on thefinished part, makes it possible to obtain a mechanical resistance inexcess of 1000 MPa, a substantial resistance to shocks, fatigue,abrasion and wear, while retaining a good resistance to corrosion aswell as a good capacity for welding, painting and gluing.

The present invention will now be further described by way a certainexemplary embodiments which are not intended to be limiting.

EXAMPLES Example 1

In a first example of implementation, a cold rolled steel sheet, 1.9 mmthick, containing by weight 0.23% carbon, 1.25% manganese, 0.017%phosphorus, 0.002% sulfur, 0.27% silicon, 0.062% aluminum, 0.021%copper, 0.019% nickel, 0.208% chromium, 0.005% nitrogen, 0.038%titanium, 0.004% boron, 0.003% calcium-has been pre-coated with analuminum-based alloy with composition 9.3% silicon, 2.8% iron, theremainder being aluminum and unavoidable impurities. According to theconditions of fabrication, namely the settings of the blowing devices onthe operating line, sheets of 120 m long and 650 mm wide with variousthickness ranges were produced.

Sheet A (according to the invention): The thickness t_(p1) and t_(p2) oneach side of the sheet was controlled to be within the range (20-33)micrometers, at every location of the two faces of the sheet, both inthe longitudinal (or rolling) direction and in the transversaldirection. Measurement was performed continuously with thickness gagesdevices relying on X-Ray emission. At every moment, the spot ofmeasurement of each gage was a circular zone of about 20 mm radius. Thesheets were afterwards cut into blanks of 1.2×0.5 m² of overalldimensions.

Sheet B (reference): On these sheets, the pre-coating thickness had awider variability since the thickness t_(p1) and t_(p2) on the two sidesof the sheet was comprised in the range (30-45) micrometers. Blanks cutout of these sheets exhibit the same pre-coating thickness.

The blanks were then submitted to heating in a furnace at T=920° C.Heating time was 3 min, with 4 mn holding time. The microstructure isthen fully austenitic. Blanks were thereafter transferred from thefurnace to a stamping press. When the elapsed time between the exit ofthe blanks from the furnace and the transfer in the stamping press ismore 10 seconds, a partial transformation from austenite was susceptibleto appear, thus reducing the mechanical resistance of the stamped part.

The blanks were directly cooled afterwards without hot stamping in orderto appreciate the eventual remelting of the coatingOn series A, no melting of the pre-coating was found. Intermetallicalloying between the pre-coating and the steel substrate occurredcompletely.On series B, the pre-coating underwent mainly alloying, but some tracesof remelting were found, particularly on the former thicker locations ofthe pre-coating. This partial remelting of the aluminium pre-coatingcontributes to the progressive fouling of the rollers in the furnace.The sheets according to the invention do not contribute to thisprogressive buildup on the rollers.

Example 2

-   -   i) Conditions according to the invention: In a second example of        implementation, a cold rolled steel sheet, 1.2 mm thick, 120 m        long and 650 mm wide, with same composition and same pre-coating        as in example 1, has been fabricated. The sheets were afterwards        cut into blanks which were heated at 920° C. for 6 mn, this time        including the heating phase and the holding time. Heating rate        V_(c) between 20 and 700° C. was 10° C./s. The blanks were        finally hot stamped and quenched in order to obtain full        martensitic structures.        The parts obtained after hot-stamping are covered by a coating,        40 micrometers thick, illustrated at FIG. 1, which has a four        layer structure. Starting from the steel substrate, the layers        are the following:    -   (a) Interdiffusion layer or intermetallic layer, 17 micrometers        thick. This layer is itself composed of two sub-layers. Hardness        HV50g ranges from 295 to 407, and the mean composition is: 90%        Fe, 7% Al, 3% Si.    -   (b) Intermediate layer, appearing darker, 8 micrometers thick.        This layer has a hardness of 940HV50g and a mean composition, by        weight: 43% Fe, 57% Al, 1% Si.    -   (c) Intermetallic layer appearing as a pale phase, 8 micrometers        thick, displaying a hardness of 610HV50g, a mean composition of        65% Fe, 31% Al, 4% Si    -   (d) Darker superficial layer, 7 micrometers thick, 950 HV50g,        with a mean composition of 45% Fe, 54% Al, 1% Si        Layers (c) and (d) are quasi-continuous, i.e. occupying at least        90% of the level corresponding to the considered layer. In        particular, layer (c) does not reach the extreme surface except        very exceptionally. Anyway, this layer (c) occupies less than        10% of the extreme surface.        ii) Conditions of reference: On the other hand, blanks with the        same base material and pre-coating parts were furnace-heated in        different conditions: The blanks were heated to 950° C. for 7        minutes, this time including the heating phase. Heating rate        V_(c) was 11° C./s. These conditions correspond to a degree of        alloying which is more important than in conditions (i)    -   In this coating, the pale intermetallic layer (c), is not        continuous and appears as to be scattered within the coating.        About 50% of this layer is present at the extreme surface of the        part. Moreover, the interdiffusion layer, 10 micrometers thick        in contact with the steel substrate is thinner than in the        previous case of FIG. 1.

Resistance spot welding was performed in the two situations i) and ii):

-   -   (i): Coating with quasi-continuous layers (c) and (d), layer (c)        occupying less than 10% of the extreme surface    -   (ii): Coating with mixed and discontinuous layers, layer (c)        occupying more than 10% of the extreme surface        Resistance spot welding was performed by superposing two parts        and joining them in the following conditions:    -   Squeeze force and welding force: 4000 N    -   Squeeze time: 50 periods    -   Welding and holding time: 18 periods respectively        In each condition, the suitable intensity range was determined        for obtaining:    -   No sputter during welding    -   Acceptable nugget size.        For the condition i), the weldability range, expressed in terms        of current intensity, is 1.4 kA. For the condition ii) the        weldability range is extremely small.

Thus, it may be seen that the coating according to the invention, yieldsmuch more satisfactory results.

Example 3

In a third example of implementation, a cold rolled steel sheet of theexample 1 was cut into blanks of 500×500 mm² which were heated at 920°C., during 6 mn, then hot stamped and cooled in tools, in suchconditions that two different cooling rates were obtained:

-   -   (A): Cooling rate: V_(A)=30° C./s    -   (B): Cooling rate: V_(B)=60° C./s        Due to the shape of he parts, different deformation levels ε        were created during hot stamping. In particular, some zones        largely strained display deformation levels higher than 30%.    -   As illustrated on FIG. 3, metallographic observations reveal        that when ε>10%, partial bainitic or ferritic transformation        occurs on parts cooled with V_(A)=30° C./s, mainly on former        austenitic grain boundaries. On the other hand, the parts cooled        with V_(B)=60° C./s display fully martensitic microstructure as        illustrated on FIG. 4. The latter structures display superior        mechanic resistance and a great homogeneity in the case of        mechanical solicitation.        Thus, even in products or parts where straining is larger than        10%, the application of cooling according to the invention        guarantees a microstructural and mechanical homogeneity.

Example 4

In a fourth example of implementation, steel castings containingdifferent values of sulfur, were elaborated. These steels were furtherhot rolled, then cold rolled steel into sheets, 2.2 mm thick. Sulfurcontent varies from 11 ppm (0.0011%) to 59 ppm (0.006%) in weight. Apartsulfur, the compositions of these different steel castings comprise inweight: 0.24% carbon, 1.17% manganese, 0.01% phosphorus, 0.25% silicon,0.045% aluminum, 0.01% copper, 0.02% nickel, 0.2% chromium, 0.04%titanium, 0.003% boron, 0.002% calcium, the remainder being iron andunavoidable impurities.These sheets were pre-coated with an aluminum-based alloy of compositioncomprising 9.3% silicon, 2.8% iron, the remainder being aluminum andunavoidable impurities.The sheets were afterwards cut into blanks which were heated at 950° C.for 5 mn, then hot stamped and cooled in tools in order to obtain a fullmartensitic structure. Mechanical resistance exceeded 1450 MPa.Specimens were extracted according to the transverse sense of rollingdirection and submitted to a bending test with alternate bending modes.The inventors put into evidence that the critical bending angle (angleat fracture) is closely related to the sulfur content of the steel: whensulfur content is lower than 0.002%, the bending angle exceeds 60°,which indicates higher ductility and energy absorption.Compact Tensile-Test type specimens were also extracted according to thetransverse rolling direction in order to measure the resistance totearing, i.e. the energy which is necessary for the initiation orpropagation of an existing crack. The results, illustrated on FIG. 6,indicate that a initiation energy higher than 18 Joules is achieved whensulfur content is lower than 0.002% in weight.As these qualities of high resistance, high energy absorption andweldability are required in automobile industry, the parts or productsfabricated according to the invention will be used with profit for suchapplications.

While the above description is clear with regard to the understanding ofthe invention, the following terms as used in the following list ofpreferred embodiments and claims have the following noted meanings inorder to avoid any confusion:

pre-coating—the material (Al or Al alloy) coated on or located on atleast a portion of the strip or sheet, etc., of base steel to form apre-coating/base composite, the composite not having been subjected toan alliation reaction between the coated Al or Al alloy material andbase steel

alliation—a reaction between the pre-coating and base steel, to produceat least one intermediate layer different in composition from both thebase steel and the pre-coating. The alliation reaction happens duringthe heat treatment immediately preceding hot stamping. The alliationreaction affects the total thickness of the pre-coating. In a highlypreferred embodiment the alliation reaction forms the following layers:(a) interdiffusion, (b) intermediate, (c) intermetallic, and (d)superficial as described above;

pre-coated steel—the pre-coating/base composite, not having beensubjected to an alliation reaction between the coated material and basesteel;

coating—the pre-coating after having been subjected to an alliationreaction between the pre-coating and base steel. In a highly preferredembodiment the coating comprises layers (a) interdiffusion, (b)intermediate, (c) intermetallic, and (d) superficial described above;

coated steel or product—the pre-coated steel or product that has beensubjected to an alliation reaction between the pre-coating and basesteel. In a highly preferred embodiment the coated steel is a strip orsheet, etc., of base steel having thereon an invention coatingcomprising layers (a) interdiffusion, (b) intermediate, (c)intermetallic, and (d) superficial described above;

blank—a shape cut from a strip.

product—a stamped blank

Thus, the present invention provides, among other things, the followingpreferred embodiments:

1-23. (canceled)
 24. A coated steel stamped product, which comprises:(a) a strip of base steel having a first side and a second side; and (b)a coating on at least one of said first side of said strip of base steeland said second side of said strip of base steel, wherein: (i) saidcoating results from the interdiffusion between said base steel, andaluminium or aluminium alloy pre-coating, and (ii) wherein said coatingcomprises, proceeding from the base steel outwards: (a) anInterdiffusion layer, (b) an Intermediate layer, (c) an Intermetalliclayer, and (d) a Superficial layer, wherein said coating has a thicknessgreater than 30 micrometers and wherein said layer (a) has a thicknessless than 15 micrometers.
 25. The coated steel stamped product accordingto claim 24, wherein the said layers (c) and (d) are quasi continuous byoccupying at least 90% of a level corresponding to each said layer andwherein less than 10% of layer (c) is present at an extreme surface ofthe product.
 26. The coated steel stamped product according to claim 24,wherein the steel in the strip comprises the following components byweight based on total weight: 0.15%<carbon<0.5%; 0.5%<manganese<3%;0.1%<silicon<0.5%; 0.01%<chromium<1%; titanium<0.2%; aluminum<0.1%;phosphorus<0.1%; sulfur<0.05%; 0.0005%<boron<0.08%; and furthercomprises iron and impurities inherent in processing.
 27. A coated steelstamped product according to claim 24, wherein the steel in the stripcomprises the following components by weight based on total weight:0.20%<carbon<0.5%; 0.8%<manganese<1.5%; 0.1%<silicon<0.35%;0.01%<chromium<1%; titanium<0.1%; aluminum<0.1%; phosphorus<0.05%;sulfur<0.03%; 0.0005%<boron<0.01%; and further comprises iron andimpurities inherent in processing.
 28. The coated steel stamped productaccording to claim 24, wherein the base steel comprises 20 ppm or lessof sulfur.
 29. The coated steel stamped product according to claim 24,wherein the ratio of titanium to nitrogen in the base steel in weight %is in excess of 3.42.
 30. The coated steel stamped product according toclaim 24, wherein the aluminum or aluminum alloy pre-coating comprisesfrom 8% to 11% silicon by weight, from 2% to 4% iron by weight, theremainder being aluminum and impurities inherent in processing.
 31. Aland motor vehicle comprising the heat treated coated steel productaccording to claim 24.